The present invention relates to a method of brazing, particularly but not exclusively to a method of furnace brazing copper.
As is well-known in the art, brazing is a method of joining two or more pieces of metal or of alloy together by means of a very thin film of a suitable brazing solder (sometimes called xe2x80x9cspelterxe2x80x9d). The brazing solder is melted and directed into the space between the pieces, and as it cools and solidifies it fuses and joins the pieces together. Brazing differs from welding in that in brazing the metal surfaces being joined are not themselves fused with the brazing solder, the melting point temperature of the brazing solder usually being considerably lower than that of the metal being joined.
In a common process, known as furnace brazing, the brazing solder is placed, either in the form of small particles or as a solid preform, on and adjacent to the joint (thus forming what is known as a xe2x80x9cbrazementxe2x80x9d); a flux may be similarly placed, and then the whole is heated to brazing temperature by being placed in a continuous type furnace where the brazing solder melts and runs by capillary action in between the pieces.
A problem in most brazing processes arises due to the cleanliness of the surfaces to be joined together; the presence of any oil, grease, oxide or any other surface contaminant can affect the flow of the molten brazing alloy along, and its adhesion on fusing to, the metal surfaces. This leads to poor capillary action which results in the creation of discontinuous and poorly-adherent joints (or xe2x80x9cdry jointsxe2x80x9d). If capillary action is poor the brazing solder will not run as far thus brazing solder must be placed closer to all the joints in a brazement increasing set-up time and expense.
There are several conventional techniques for addressing this cleanliness problem. For example, the surfaces can be cleaned and degreased with solvents prior to brazing, however this is time-consuming and relies on the use of costly and environmentally-harmful solvent materials. Such methods are also usually rather ineffective in removing oxides from the surfaces to be joined, particularly as further oxidation can occur as the brazement is heated to the brazing temperature. Consequently, it is common to carry out brazing in an inert gas atmosphere in order to prevent such further oxidation. Heretofore therefore, in order to improve the standard of brazed joints, efforts have been directed towards the substantial elimination of the surface contaminants known to adversely affect brazing time and brazing adhesion.
It is an object of this invention to address the problems associated with the prior art with the aim of producing a brazing method which provides an improved brazed joint and/or to reduce the set-up time, expense or other disadvantageous results of conventional methods.
The present invention provides a brazing method for joining two or more pieces of metal together comprising running a molten brazing solder into a space between the pieces, characterized by the step of heating at least the adjacent surfaces of the pieces to be joined to brazing temperature in a reducing atmosphere and by the step of preheating at least the adjacent surfaces to a predetermined temperature below the brazing temperature but above the temperature where oxidation readily occurs in an oxidizing atmosphere prior to said heating to brazing temperature.
Preferably this predetermined temperature is between about 400xc2x0 C. and 700xc2x0 C., and typically about 600xc2x0 C. for copper.
Contrary to the generally-accepted convention, we have found that the deliberate oxidation of the surfaces to be brazed, followed by reduction just prior to the brazing operation (which reduction atmosphere provides a fluxing action, to remove the oxides existing on the surface of the metal) produces an excellent combination of braze gap filling, strongly adherent bonding between the fused brazing solder and the metal and final surface appearance of the brazed joint, and a combination which represents an improvement over the prior art.
It is believed that the advantages arising from the method of the invention are due to the fact that a freshly reduced surface has a higher surface xe2x80x98activityxe2x80x99 than an already oxide-free surface, and thus is xe2x80x98wettablexe2x80x99 by the brazing alloy resulting in better gap filling. Although the surface of a joint brazed in accordance with the invention may have less xe2x80x9cglossxe2x80x9d than one brazed in a conventional manner, this is only really of any relevance in applications in which surface decoration rather than joint integrity is important.
Preferably, the method comprises supplying an inert gas to the furnace to provide a furnace atmosphere predominantly composed of said inert gas, an oxidizing gas being introduced into the preheating zone and a reducing gas being introduced into the heating zone.
The predominance of inert gas prevents further oxidation of the surfaces to be joined and the concentration of oxidizing and reducing gases to be controlled accurately and consistently. For example, copper brazing is typically carried out in an exothermically generated gas atmosphere. Such atmospheres contain hydrogen, carbon monoxide, carbon dioxide and water vapour as well as nitrogen. The hydrogen and carbon monoxide present in the atmosphere would react with any oxygen (as air) added to a pre-oxidation zone creating water vapour and carbon dioxide which are not oxidizing to copper. It would be necessary therefore to make sufficient oxygen addition to convert all the hydrogen and carbon monoxide in the pre-treatment zone plus a small excess to give the required oxidation. However, as exothermically generated gas used for brazing typically contains a combined hydrogen and carbon monoxide content of anything between 2% and 22%, the control of a small excess of oxygen would be difficult especially as the hydrogen and carbon monoxide are not themselves controlled. The best results are therefore likely to be achieved using an inert nitrogen based atmosphere system with air added to the pre-treatment zone for the oxidation step and hydrogen to the brazing zone for the reduction step.
Preferably the oxidizing gas is introduced so as to maintain the free oxygen level in the preheating zone in the range 0.2% to 0.3%, since such a range has been found to provide optimum braze joint characteristics. Such an atmosphere at 600xc2x0 C. is sufficiently oxidizing to ensure the removal of any residual organic material from the surface of the copper and to form the necessary adherent oxide film.
We have found that treatment of copper in lower oxygen atmospheres for short periods does not result in visible oxide film (probably due to a relatively slow oxidation site nucleation rate at low oxygen levels). Even at higher oxygen levels the oxide layer is very thin and is best described by the copper:oxygen ratio of the surface layer.
A method in accordance with this invention will now be described, by way of example and with reference to the accompanying drawing of a test piece used to provide the comparative results below.
Referring to FIG. 1, a simple brazing test piece was devised to give a consistently increasing braze gap but which did not require accurate machining. The test piece consisted of two 300 mm lengths of 15 mm diameter copper tube 2 which were bound together with copper wire at each end (not shown). At a distance of 250 mm from one end a 1 mm diameter wire 4 was placed between the tubes producing a constantly increasing and consistent gap. A 75 mm length-6 of braze alloy (Phos0 (BS1845:1984 CP3) supplied by Thessco Limited) weighing 2g which was calculated to be sufficient to fill 75% of the joint length was placed at the point where the tubes touched.